There has heretofore been known a multistage pump having a plurality of intermediate casings formed by press-forming a steel plate, and impellers housed in respective intermediate casings. FIG. 7 is a vertical cross-sectional view showing a structure including intermediate casings in a conventional multistage pump. As shown in FIG. 7, impellers 101 are housed in the intermediate casings 100. The intermediate casing 100 has a cylindrical side portion 102, a fitting portion 103 formed substantially into a sigmoid shape, and a bottom portion 104 extending radially inward from the fitting portion 103.
An axial end face 102a and an inner surface 102b of a cylindrical side portion 102 of an adjacent intermediate casing are held in contact with the fitting portion 103. The inner surface 120b of the cylindrical side portion 102 of the adjacent intermediate casing and the fitting portion 103 having substantially a sigmoid shape form a recess in which an O-ring 110 is fitted. The fitting portion 103 of the intermediate casing is formed by buckling a steel plate through press forming.
However, when the fitting portion 103 of the intermediate casing is formed by press forming, the shape of the recess for the O-ring is unlikely to be fixed due to variations of the hardness or the thickness of a casing material, and thus it is difficult to obtain a stable shape of the fitting portion. Further, a die for press forming is required to have high accuracy. Furthermore, since a special O-ring 110 that is fitted into the shape of the fitting portion 103 is required, cost rises problematically.